For small size and multi-functions of electric appliances, particularly for lightness, thinness, shortness and small size of portable appliances, there is a demand for high density of a circuit board used in electric appliances and generally, a circuit board is made in a multi-layered structure to meet the demand. Also, a flexible printed circuit base board may be used to be installed in a limited space or a circuit with a narrow line width may be used to produce a large amount of circuits in the same space. Meanwhile, considering the environmental problems caused by soldering for the multi-layered structure, concern about an adhesive to produce the multi-layered structure without using lead is increased and there is a demand for an adhesive with high adhesion, high thermal resistance and low moisture absorption for a circuit board.
The conventional metallic laminate comprising a polyimide film and a metal foil adhered to each other by an acryl or epoxy adhesive is not adequate for a circuit board requiring a multi-layered structure, flexibility, high adhesive power and high thermal resistance. Therefore, there has been developed an adhesive free type flexible metallic laminate which is produced by directly adhering a polyimide and a metal foil without using an adhesive.
Meanwhile, for a high density circuit, a circuit board of a double-sided metallic laminate comprising metal foils laminated at both sides of a polyimide resin is preferred.
Japanese Patent Laid-Open Publication No. Heil-244,841 discloses a method for preparing a double-sided metallic laminate for a printed circuit board using a thermoplastic polyimide having a glass transition temperature exceeding 250° C. Therefore, it has problems in that the lamination of the thermoplastic polyimide on a metal foil is performed under a condition including a high temperature of over 350° C. and a high pressure of over 50 kg/cm2.
Also, U.S. Pat. No. 5,112,694 discloses a double-sided metallic laminate, in which a single layer of a polyimide is coated on a metal foil and another metal foil is laminated thereto at a temperature of over 380° C. and a pressure of 20 kg/cm or more. However, this double-sided metallic laminate has problems in that it should be subjected to a high temperature and high pressure condition.